Xbit Labs Presents: LCD Monitors Testing Methodology Indepth. Page 2

This article is an explanation of methods we use to test LCD monitors. It provides a list of tested parameters with remarks on their meaning, and a description of the measuring instruments. We will also offer you reference to all our previous articles discussing the testing approach we use, so that you could have all the clues to our extensive testing techniques in one place.

Color Reproduction

Color Gamut

The term “color gamut” defines how many of the colors a human eye can perceive can be reproduced by the monitor. Our eye can recognize the so-called optical spectrum – electromagnetic radiation with the wave lengths varying between 380 and 700 nm. Our brain perceives it as a spectrum of primary colors from purple to red. Other colors are created as a mix of radiation waves of different lengths, while the continuous spectrum (the one with waves of all lengths in it) is perceived as white light.

It is technically impossible to design a device that would be able to synthesize in real time a random spectrum to create desired colors, but luckily, we don’t need an exact replica of the original spectrum in order to see this or that color. Our eye features only three types of color receptors (red, green and blue) and our brain uses their signals to determine the color. It means that all we need to do is stimulate these receptors in proper proportion, which can be done via three sources of monochromatic light (red, green and blue) with controlled intensity.

Unfortunately, we will only be able to almost cover the color gamut our eye can perceive: there will remain some smaller areas in the color range that we can actually see, but our device cannot reproduce. To even greater regret, the less monochromatic the spectrum produced by each light source of our device is, the larger is the range of colors it can never reproduce.

They usually use the so-called CIE-diagram to describe illustratively the color gamut. A horseshoe shaped area on it represents the entire color spectrum available to the human eye. Around the edges of this “horseshoe” there are primary colors, closer to the center – mixed colors up to white. If we place the dots corresponding to color coordinates of the three light sources of our device (in a specific example these would be coordinates of each of the three subpixels of our monitor’s matrix RGB-filter), they will form a triangle. This triangle indicates the range of colors that the device in question can reproduce. The size of this area is called color gamut.

The size of this area has no connection to the color representation in the monitor: 18 bit (262 thousand colors), 24 bit (16.7 million colors), etc. These numbers only show how precisely we can define the color inside the triangle, but they don’t allow us to get beyond its boundaries. To expand the triangle we will need to somehow change the monitor luminosity spectrum thus shifting the triangle vertex coordinates. Contemporary LCD monitors use different types of backlight to achieve this effect: regular CCFL bulbs provide the smallest color gamut, while the new bulbs with improved phosphors provide considerably larger color gamut. The maximum color gamut can be achieved if the backlight bulbs get replaced with light emitting diodes.

There are a few types of color gamut. The most frequently mentioned ones are sRGB, AdobeRGB and NTSC (they correlate quantitatively as follows: sRGB < AdobeRGB < NTSC). sRGB is standard color gamut for CRT and LCD monitors, most of the models out there feature it. To be more exact, though, contemporary LCD panels usually exceed sRGB a little in the green color spectrum, you can see it on the picture above, but overall the difference is not dramatic. sRGB gamut equals 72% of the NTSC gamut, and the actual gamut of most LCD monitors is around 75% NTSC. Monitors with better backlight bulbs can reach almost 97% NTSC gamut, while monitors with LED backlight already hit 114% NTSC and this number will most likely continue to grow.

How can we benefit from increasing color gamut? The monitor can reproduce clearer primary colors that the models with smaller gamut cannot ever offer: “clear” red has less yellow, “clear” blue – less green, etc. It would be incorrect to claim that increase in color gamut improves color reproduction precision. Firstly, the latter is currently determined by multiple parameters and large color gamut is primarily a nice addition to them, but in no way a determinative factor. Secondly, images (or to be more exact – image files) usually contain no info on the color gamut they have been created for, which means that a priori they are optimized for sRGB as the most widely spread color gamut type. However, since the “clear” green color will be different in monitors with sRGB and NTSC gamut, then the latter will actually reproduce the images optimized for the former one, hence the image will not be reproduced correctly. And vice versa.

However, if we are talking about professional application, the increasing color gamut may cause some other less obvious issues, which we’d better leave for professionals to handle. For home users larger color gamut brings clearer more natural-looking colors on the screen of your monitor. Although I have to stress once again that gamut itself doesn’t guarantee precise color reproduction and is a nice addition to the other monitor specifications that should be on a high level as well.